The present invention relates to an electronic balance, and more particularly to an electronic balance in which a work of adjusting a four-corner error can be easily conducted or can be eliminated. The invention can be applied to a so-called electronic balance in which an electromagnetic force balancing mechanism that generates an electromagnetic force opposing a load on a weighing dish, and that detects the load on the weighing dish on the basis of the magnitude of the generated electromagnetic force is used as a load detecting mechanism, and also to a so-called electronic scale which has a load detecting mechanism including a load sensor such as a load cell.
Usually, an electronic balance or an electronic scale comprises a load detecting mechanism which outputs a signal corresponding to a load applied to a load sensing portion, and has a structure in which the load detecting mechanism supports a weighing dish via a dish support. From an output of the load detecting mechanism, calculating means such as a microcomputer calculates the value of the load on the weighing dish.
In such an electronic balance or an electronic scale, usually, a configuration including a Roberval mechanism (also called as a parallel guide) is often employed as a load detecting mechanism in order to eliminate an error due to the position of the center of gravity of a load on a weighing dish, i.e., a four-corner error. A Roberval mechanism has a structure in which a stationary column fixed to a balance base or the like, and a movable column attached to a dish support are coupled to each other via two upper and lower parallel beams that comprise flexible portions in both ends, respectively. Even when a load is applied to a position deviated from the center of the weighing dish in an arbitrary direction, the load detecting mechanism is enabled to output a given signal corresponding to the load by adjusting the parallelism between the upper and lower beams, in other words, by adjusting the vertical gaps between the flexible portions which are disposed in the ends of the upper and lower beams, so as to be equal to each other. As a result, a four-corner error can be eliminated.
A work of mechanically adjusting the parallelism between the upper and lower beams in a Roberval mechanism so as to eliminate such a four-corner error is conducted while actually changing the load applying position on a weighing dish after a balance is assembled. The work requires special technique. Furthermore, phenomena that a four-corner error does not appear evenly in a diagonal direction of the weighing dish, and that a four-corner error fails to be in proportional to the magnitude of the load and the deviation distance (the distance from the center of the weighing dish to the center of gravity of the load) occur. In such a case, it is substantially impossible to strictly eliminate a four-corner error by mechanical adjustment using a Roberval mechanism.
The invention has been conducted in view of such circumstances. It is an object of the invention to provide an electronic balance in which influences due to a four-corner error can be eliminated from an indicated value of weighing without strictly conducting mechanical adjustment, so that correct indication can be always performed.
In order attain the object, the electronic balance of the invention is an electronic balance comprising: a load detecting mechanism which supports a weighing dish via a dish support, and which outputs a signal corresponding to a load on the weighing dish; and calculating means for calculating a value of the load on the weighing dish by using an output of the load detecting mechanism, wherein the dish support comprises four cantilever beams which respectively elongate in four directions in a horizontal plane, vicinities of tip ends of the cantilever beams respectively supporting corners of the weighing dish, and the electronic balance further comprises: deflection amount detecting means for individually detecting a deflection amount of each of the cantilever beams; and four-corner error correcting and calculating means for calculating a position of a center of gravity of the load on the weighing dish by using a result of the detection of deflection amounts of the cantilever beams, for, by using previously stored relationships between a position of a center of gravity of a load and a four-corner error, correcting the output of the load detecting mechanism, and for causing the corrected output to be used in the calculation of the value of the load by the calculating means.
In the invention, the position of the center of gravity of the load placed on the weighing dish is detected, and the output of the load detecting mechanism is corrected in accordance with the detected position, whereby a four-corner error can be eliminated without strictly conducting mechanical adjustment by means of a Roberval mechanism.
When cantilever beams elongating respectively in four directions are disposed on the dish support that receives the weighing dish and the weighing dish is supported by the vicinities of the tip ends of the cantilever beams, each of the cantilever beams is deflected by an amount corresponding to the position of the center of gravity of the load on the weighing dish. The deflection amounts of the cantilever beams are respectively detected by the deflection amount detecting means. From ratios of the deflection amounts, the position of the center of gravity of the load on the weighing dish can be obtained.
Furthermore, the four-corner error (rate) of the output of the load detecting mechanism is uniquely determined in accordance with the position of the center of gravity of the load on the weighing dish.
Therefore, relationships between the position of the center of gravity of a load onto the weighing dish and a four-corner error are previously measured and stored. In an actual use of the electronic balance, when a sample or the like is placed in any position on the weighing dish in order to weigh the load due to the sample, the position of the center of gravity of the sample on the weighing dish is obtained by calculation on the basis of results of detections of the deflection amounts of the cantilever beams which are conducted by the deflection amount detecting means. By using a result of the calculation and the contents of the above-mentioned storing, it is possible to know a four-corner error contained in the output of the load detecting mechanism. A correction calculation of eliminating an amount corresponding to the error from the output of the load detecting mechanism is performed, and a result of the calculation is used in the calculation of the value of the load by the calculating means. Therefore, although the actual output of the load detecting mechanism contains a four-corner error, the indicated value of weighing is a correct value which does not contain the four-corner error.